Heat exchanger for heating of fuel cell combustion air

ABSTRACT

A heat exchanger for a solid-oxide fuel cell assembly. A plurality of parallel tubes conveys fuel cell stack exhaust gas from a first manifold means to a second manifold means. The tubes are highly corrugated to increase the wall area and decrease the wall thickness. The tubes are disposed in a jacket through which is passed incoming air to be heated. The tubes may be linear between two manifolds, or they may be curved such that the first and second manifold functions are accommodated within a single component.

TECHNICAL FIELD

[0001] The present invention relates to solid-oxide fuel cells (SOFCs);more particularly, to heat exchangers for heating incoming combustionair in an SOFC assembly; and most particularly, to an improved heatexchanger for increasing heat exchange efficiency and reducing heatexchanger manufacturing cost and complexity.

BACKGROUND OF THE INVENTION

[0002] Fuel cells combining hydrogen and oxygen to produce electricityare well known. A known class of fuel cells includes a solid oxideelectrolyte layer through which oxygen anions migrate; such fuel cellsare referred to in the art as “solid-oxide” fuel cells (SOFCs).

[0003] In some applications, for example, as an auxiliary power unit(APU) for an automotive vehicle, an SOFC is preferably fueled by“reformate” gas, which is the effluent from a catalytic gasolineoxidizing reformer. Reformate typically includes amounts of carbonmonoxide (CO) as fuel in addition to molecular hydrogen. The reformingoperation and the fuel cell operation may be considered as first andsecond oxidative steps of the liquid hydrocarbon, resulting ultimatelyin water and carbon dioxide. Both reactions are exothermic, and both arepreferably carried out at relatively high temperatures, for example, inthe range of 700° C. to 1000° C.

[0004] Air enters an SOFC fuel cell at ambient temperature and desirablyis preheated before being sent to the fuel cell stacks. A convenient andeconomical way to heat the air is by abstracting heat via a heatexchanger from the fuel cell exhaust which exits the fuel cell combustorat about 950° C. In the prior art, a typical heat exchanger employed forthis purpose is of a well known plate-and-frame design wherein aplurality of heat-exchange modules is assembled as a stack. A pluralityof alternating hot and cold gas flow spaces are separated by heattransfer plates. A typical prior art heat exchanger for use in an SOFCmay comprise more than 100 individual plates and frames and can requiremore than 200 feet of brazing to seal the edges of all the modules, andis thus complicated and expensive to fabricate.

[0005] What is needed is an efficient heat exchanger for an SOFC whereinthe number of components and fabrication costs are significantlyreduced.

[0006] It is a principal object of the present invention to reduce thecost and complexity of an SOFC heat exchanger.

SUMMARY OF THE INVENTION

[0007] Briefly described, a heat exchanger for a solid-oxide fuel cellassembly includes a plurality of parallel tubes for conveying a firstgas, preferably a hot gas, from a first manifold means to a secondmanifold means. The only brazing required is to attach each tube to eachmanifold. Preferably, the tubes are highly corrugated in bellows-likeform to increase the wall area and decrease the wall thickness. Thetubes are disposed in a jacket through which is passed a second gas,preferably a cool gas. The tubes may be linear between two manifolds, orthey may be curved such that the first and second manifold functions areaccommodated within a single component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other features and advantages of the invention will bemore fully understood and appreciated from the following description ofcertain exemplary embodiments of the invention taken together with theaccompanying drawings, in which:

[0009]FIG. 1 is an isometric view from the front, partially exploded, ofa prior art plate-and-frame heat exchanger;

[0010] FIG.2 is an exploded isometric view from the front of a firstembodiment of a heat exchanger in accordance with the invention;

[0011]FIG. 3 is an exploded isometric view from the front of a secondembodiment of a heat exchanger in accordance with the invention; and

[0012]FIG. 4 is an exploded isometric view from the front of a thirdembodiment of a heat exchanger in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Referring to FIG. 1, a prior art heat exchanger 10 comprises aplurality of alternating hot and cold fluid flow spaces formed byalternating rectangular plates 12 and frames 14. In an SOFC assembly,the hot fluid is hot exhaust gas from the fuel cell stack and the coldfluid is combustion air entering the assembly. Each plate and frame hasperforated extensions 16 at all four edges such that, when assembledinto a solid stack 18, the perforations define inlet and exhaustmanifolds 20,22 for a first fluid 23 flowing across the plates in afirst direction 24, and inlet and exhaust manifolds 26,28 for a secondfluid 29 flowing across opposite sides of the plates in a second andorthogonal direction 30. The extensions of first frames 14 a are open ontheir inner edges to permit access of first fluid 23 from manifold 20 tofirst sides of plates 12, and the extensions of second frames 14 b areopen on their inner edges to permit access of second fluid 29 to theopposite sides of plates 12. It will be seen that the sequence of platesand frames 12-14 a-12-14 b represents a modular repeat, and that thefull stack is simply a plurality of such modular repeats, the number ofmodules being as desired for a particular heat exchange requirement. Inprior art SOFC heat exchanger 10, the number of modules is typicallyabout 25, requiring 100 or more components. After the entire stack ofplates and frames is assembled, the edges of all plates and frames aresealed as by brazing to prevent fluid leakage from the heat exchanger.

[0014] Referring to FIG. 2, a first embodiment 110 of an SOFC heatexchanger in accordance with the invention includes a plurality ofparallel metal tubes 112 having ends 113 (one set of ends not visible).Lower end plate 114 is provided with a plurality of openings 116arranged in a pattern, each opening being surrounded by a lip 118 forreceiving a first end (not visible) of a tube 112. Upper end plate 120is similarly provided with openings 122 and lips (not visible). Theexemplary pattern of openings and tubes is four staggered rows of fivetubes each. Obviously, other patterns are possible within the scope ofthe invention. The tubes are attached to the end plate lips as bybrazing.

[0015] Metal tubes 112 preferably are axially corrugated as byhydro-forming into bellows form such that the surface area of each tubeis substantially greater than the surface area of a non-corrugated tubehaving equal length and diameter. Preferably, the surface area is atleast doubled. In addition, the bellows-forming process, which is wellknown in the art, causes thinning of the tube wall. As a result, thethermal conductance of heat exchanger 110 can be as much as 200% greaterthan that of prior art heat exchanger 10 of comparable size.

[0016] Preferably, tubes 112 are formed of a nickel-based hightemperature alloy, for example, Inconel 625.

[0017] A base plate 124 has a planar upper surface 126 for matingagainst a 20 planar lower surface 128 of lower end plate 114. Surface126 is relieved in three areas. One is a central well 130 defining anintermediate manifold for mating with the central two rows of tenopenings 116; the other two are lateral wells 132 a,132 b, each of whichdefines an intake and exhaust manifold, respectively, which mates with arespective lateral row of five openings 116. Well 132 a is provided withslots 134 extending through plate 124 for mating with a supply such asan intake manifold (not shown) of a first fluid 23, preferably the hotexhaust gas from the fuel cell stack. Well 132 b is provided withsimilar slots 136 for mating with a return pathway through an exhaustmanifold (not shown) for first fluid 23.

[0018] A cover plate 138 has a planar lower surface 140 for matingagainst a planar upper surface 142 of upper end plate 120. Surface 142is relieved in two wells 144 a,144 b, each of which defines a first andsecond crossover manifold, respectively. Each well contains tworespective lateral rows of five openings 122. Wells 144 a,144 b areseparated by a median 146.

[0019] The result of this arrangement is an “M” shaped path for gasthrough five parallel tube assemblies. A first gas (fuel cell exhaustgas) at a first starting temperature enters through slots 134, passesthrough openings 116 into the first staggered row of five tubes 112,passes upwards through openings 122 into crossover manifold 144 a,passes downwards through openings 122 into the second staggered row offive tubes 112, passes through openings 116 into central well 130,passes upward through openings 116 into the third staggered row of fivetubes 112, passes upward through openings 112 into second crossovermanifold 144 b, passes downward through openings 112 into the fourthstaggered row of five tubes 112, passes downward through openings 116into lateral well or manifold 132 b, and passes out of heat exchanger110 via slots 136.

[0020] Referring still to FIG. 2, tubes 112 and upper plate 120 aresurrounded by a jacket 150 defining a jacketed space 152 between jacket150 and the walls of tubes 112. Jacket 150 is sealed to cover plate 138and to lower end plate 114. Lower end plate 114 is attached to baseplate 124 as by bolts (not shown) through bores 154. Lower end plate 114and base plate 124 are provided with slots 158,160. A second gas at asecond starting temperature (air to be heated) enters through inletslots 158, passes into jacketed space 152, passes around corrugatedtubes 112 abstracting heat therefrom, and exits through exhaust slots160.

[0021] Referring to FIG. 3, a second embodiment 210 is identical in gasflow path to embodiment 110 but is substantially simplified inconstruction. The “M” shaped flow path is clearly visible in fivestaggered M-tubes 212 having ends 213. Each M-tube 212 preferably iscorrugated along its four linear portions as shown. Upper plate 120 andcover 138 are eliminated, their functions being cast into a closedjacket 250 conformable with M-tubes 212. Further, lower end plate 214 issimplified to have only ten openings 216 rather than twenty openings 116as in embodiment 110. The total brazing required between tubes andplates is reduced from forty joints to ten. When tubes 212 are one-halfinch in diameter, the total length of brazing required is about 15inches, as compared to 200 inches required for prior art exchanger 10 or60 inches for first embodiment 110. referably, bottom plate 214 isprovided with a plurality of attached fins 280 disposed adjacent M-tubes212 for improving air flow around the tubes. Base plate 224 issimplified to eliminate central well 130 from embodiment 110.

[0022] The “M” flow path indicated in first and second embodiments110,210 can give rise to undesirably high back pressures because of therelatively long flow path. Referring to FIG. 4, a third embodiment 310reduces the flow path by half, albeit at a cost of sixty joints as infirst embodiment 110. The basic flow arrangement provided by the twentycorrugated tubes 312 (shown for clarity via a cutout in jacket 350),having tubular inserts 313, is ten parallel U-shaped flow paths insteadof five M-shaped flow paths. Lower end plate 314 is substantiallyidentical with plate 114 in FIG. 2. Base plate 324 is configured asessentially a frame having two openings 332 a,332 b which become inletand exit chambers when plate 324 is disposed between a mounting manifold(not shown) and plate 314. Upper end plate 320 is welded to tubes 312 asin FIG. 2, and a single upper manifold space 380 is provided by a cutoutin a new spacer frame element 382 into which tubes 312 debouch. Coverplate 338 is similar to cover plate 138. The flow path then is simplyfrom inlet opening 332 a upwards through the forward ten tubes 312 intomanifold space 380, then downwards through the rear ten tubes 312 intoexhaust opening 332 b. Jacket 350 may be substantially identical withjacket 150.

[0023] A potential drawback of flowing a gas through corrugated tubingis stagnation of gas within the recesses of the corrugations. Referringstill to FIG. 4, each tube 312 preferably is provided with an internalspiral turbulator 355 which is installed into the tube prior to brazing.(For purposes of clarity, each turbulator is shown partially removedfrom the respective tubes). Turbulator 355 is formed from sheet metal,preferably a high-temperature alloy, and twisted through an axial anglesuch as 1800 about its axis. The turbulator induces a swirling flow ofgas through the tube, promoting flushing of gas from the corrugationrecesses.

[0024] While the invention has been described by reference to variousspecific embodiments, it should be understood that numerous changes maybe made within the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

What is claimed is:
 1. A heat exchanger, comprising: a) at least onecorrugated tubular element, having first and second ends, for conveyinga first fluid at a first starting temperature; and b) jacket meanssurrounding said at least one tubular element for conveying a secondfluid at a second starting temperature around said at least one tubularelement, whereby heat is transferred between said first fluid and saidsecond fluid.
 2. A heat exchanger in accordance with claim 1 furthercomprising first and second connecting means attached to said first andsecond ends, respectively, for conducting said first fluid into and outof said at least one tubular element.
 3. A heat exchanger in accordancewith claim 1 further comprising at least one turbulator disposed withinsaid at least one tubular element for agitating said first fluidtherein.
 4. A heat exchanger in accordance with claim 1 wherein said atleast one corrugated tubular element has a surface area at least twicethat of a non-corrugated tubular element of equal length and diameter.5. A heat exchanger in accordance with claim 1 wherein said first fluidand said second fluid are both gases.
 6. A heat exchanger in accordancewith claim 1 wherein said first fluid is fuel cell exhaust and saidsecond fluid is air.
 7. A heat exchanger in accordance with claim 1wherein said at least one tubular element is selected from the groupconsisting of linear, U-shaped, and M-shaped.
 8. A heat exchanger inaccordance with claim 2 further comprising a plurality of said tubularelements arranged for parallel flow of said first fluid therein betweensaid first and second connecting means.
 9. A heat exchanger inaccordance with claim 2 wherein said first and second connecting meansinclude first and second end plates.
 10. A heat exchanger in accordancewith claim 2 wherein said first and second connecting means arecomprised in a single end plate.
 11. A heat exchanger in accordance withclaim 8 comprising an array of twenty of said tubular elements arrangedinto five M-shaped flow paths for said first fluid.
 12. A heat exchangerin accordance with claim 8 comprising an array of twenty of said tubularelements arranged into ten U-shaped flow paths for said first fluid. 13.A fuel cell assembly comprising a heat exchanger for exchanging heatbetween fuel cell exhaust gas and incoming air, said exchanger includingat least one corrugated tubular element, having first and second ends,for conveying said exhaust gas at a first starting temperature, andjacket means surrounding said at least one tubular element for conveyingsaid incoming air at a second starting temperature around said at leastone tubular element, whereby heat is transferred between said exhaustgas and said incoming air.